The present invention relates to a manufacturing method and a manufacturing device of a green compact having a through-hole at a part connected continuously to a base part or a green compact having a pair of leg parts connected continuously at a base part in which a part between both leg parts is an undercut part.
Conventional machine parts include P/M (powder metallurgy) product shown in FIG. 1(a), FIG. 1(b), and FIG. 3(a). The product shown in FIG. 1(a) comprises a rack 2 at an end of a base part 1 and has an axis hole 3 penetrating a part connected continuously to the base part 1 formed therein. The product shown in FIG. 1(b) comprises a hook part 5 at an end of an arm 4 extending from the base part 1 and has the axis hole 3 penetrating the part connected continuously to the base part 1 formed therein.
On the other hand, the product shown in FIG. 3(a) is composed of a hook part 22 provided at the front end of a rod part 21 and a pair of leg parts 24, 24 connected continuously at a base part 23 which is the rear end of the rod part 21, and has an undercut part 25 between the leg parts 24, 24.
Methods for manufacturing such a machine part having the axis hole 3 or the undercut part 25 by P/M include a method in which a green compact in a shape without the axis hole 3 or the undercut part 25 is first sintered, and then, the axis hole 3 is provided by drilling work or the undercut part 25 is provided by cutting work and the like, and a method in which the axis hole 3 or the undercut part 25 is provided at the same time of the powder compaction.
As described in the former method, in a case of performing the drilling work or the cutting work on the sintered compact, the powder compacting is performed using a die having a die hole in a shape matching the contour of a desired molded body in a vertical direction and a pair of punches inserted into the die hole from the upside and underside of the die. First, material powder is filled in a cavity which is formed of the die hole and a lower punch inserted into the die hole from the underside of the die. Next, the material powder is pressurized and compressed by the upper and lower punches to a prescribed thickness and the obtained green compact is taken out from the upside of the die hole. The green compact is sintered, and thereafter, the axis hole 3 is formed by the drilling work or the undercut part 25 is formed by the cutting work. In this method, however, the drilling work or the cutting work is time-consuming and requires an extra cost.
On the other hand, in a case of forming the axis hole 3 or the undercut part 25 at the same time as the time of powder compacting as described in the latter method, a powder compacting device is utilized which comprises, in addition to the die and the upper and lower punches, a horizontal punch or a core movable in a direction perpendicular to a pressurizing direction for shaping the axis hole 3 or the undercut part 25.
As for the powder compacting device comprising the horizontal punch for shaping the axis hole 3 or the undercut part 25, a method is known in which the cavity formed of the die hole and the lower punch is filled with the material powder while being penetrated by the horizontal punch, the material powder is compressed to the prescribed thickness by the upper and lower punches, the horizontal punch is pulled out from the obtained green compact, and the green compact is taken out from the upside of the die hole. In this manufacturing method, since the cavity is filled with the material powder while being penetrated by the horizontal punch, the material powder is not uniformly filled into a lower part than the horizontal punch in the cavity, and it is sometimes impossible to shape a green compact especially when it has a shape in which thickness of a periphery of the axis hole 3 or the undercut part 25 (the base part 1 or the leg part 24) is thin.
As for the manufacturing method in which a core in a shape corresponding to the shape of the axis hole 3 or the undercut part 25 is disposed in the cavity in advance, the cavity with the core disposed therein is filled with the material powder, the upper punch is inserted under pressure from an opening of the cavity, and the material powder is pressurized and compressed so as to obtain the green compact. The core is removable from the pressurized and compressed green compact and, by pulling it from the compacted body, the green compact having the axis hole 3 or the undercut part 25 can be obtained. In this manufacturing method, however, it is difficult to obtain the uniform density of the material powder in a lower part and in an upper part than the core when the material powder is filled.
Another method is also known in which the cavity formed of the die hole and the lower punch is filled with the material powder, the material powder is pressurized and compressed by the upper and lower punches, and the axis hole 3 or the undercut part 25 is punched out by a punch-out punch. However, in this manufacturing method, when the green compact obtained by the pressurization and compression is punched out by the punch-out punch, chipping, breakage or the like may be caused in the green compact.
Moreover, if a part of the periphery of the axis hole 3 or the undercut part 25 (the base part 1 or the leg part 24) becomes thin due to the forming of the axis hole 3 or the undercut part 25 as shown in FIG. 1(b) or FIG. 3(a), both of the manufacturing methods using the core or the punch-out punch have a disadvantage that breakage may be caused in the thin part when the green compact with the axis hole 3 or the undercut part 25 formed therein is taken out. In a case in which the undercut part 25 is formed, there is also a disadvantage that deformation such as warpage may occur in the leg part 24 in sintering which is performed after the powder compaction.
In order to solve these disadvantages, an object of the present invention is to provide a manufacturing method in which a green compact is having a through-hole or an undercut part can be easily manufactured without causing breakage.
Another object of the present invention is to provide a manufacturing device appropriate for the manufacturing method of the green compact having the through-hole or the undercut part.
To achieve these objects, the manufacturing method of the green compact according to the present invention is characterized in that it comprises a step of filling material powder into a cavity formed of a die having in a vertical direction a die hole in a shape matching a contour of the green compact including a through-hole at a part connected continuously to a base part and a lower punch inserted into the die hole from the underside of the die, a step of obtaining a preform by temporarily compressing the material powder filled in the cavity by means of an upper punch inserted from the upside of the die into the die hole and the lower punch, a step of punching out a shape of the through-hole by inserting a punch-out pin having a shape corresponding to a cross-section of the through-hole into the preform, a step of obtaining a green compact by pressurizing and compressing the preform by means of both of the upper and lower punches in a state in which the punch-out pin is being inserted, a step of pulling out the punch-out pin from the green compact, and a step of taking out the green compact from the cavity.
According to the manufacturing method of the present invention, the lower punch is first inserted into the die hole from the underside of the die so that the cavity is formed of the lower punch and the die hole.
The material powder is then filled in the cavity. When the material powder is filled, the upper punch is inserted into the die hole from the upside of the die and the material powder is temporarily compressed between the upper and lower punches so that the preform is obtained. On this occasion, since a core is not disposed in the cavity, the material powder can be filled at uniform density.
The punch-out pin is then inserted into the preform. On this occasion, since the material powder is temporarily compressed as described above, the shape of the through-hole can be easily punched out by the punch-out pin.
Thereafter, the preform is pressurized and compressed by both of the upper and lower punches while the punch-out pin is being inserted into the preform. As the punch-out pin works as a core, the green compact having the through-hole in a shape matching the contour of the punch-out pin is obtained. The preform is obtained by temporarily compressing the material powder which is filled at the uniform density. Therefore, by further pressurizing and compressing the preform, a possibility that the material powder upper than the punch-out pin and that lower than the punch-out pin have different density can be prevented even if the punch-out pin is being inserted.
The punch-out pin is then pulled out from the obtained green compact and the green compact is taken out so that a completed product can be obtained.
The manufacturing method of the green compact according to the present invention is also characterized in that it further comprises a step of sintering the green compact taken out from the cavity and a step of forming a pair of leg parts connected continuously at the base part and an undercut part formed between both of the leg parts by cutting off a part of a peripheral wall which forms the through-hole of the sintered compact.
Although a thin part is formed around the through-hole of the taken-out green compact, the thin part itself is the peripheral wall of the through-hole and connected at a part facing the base part, and thereby deformation is limited in the sintering and warpage or the like can be securely prevented from occurring.
Thus, the green compact having the pair of leg parts connected continuously at the base part and the undercut part formed between both of the leg parts can be obtained by sintering the taken-out green compact and thereafter, cutting off the part of the peripheral wall which forms the through-hole.
If the pair of leg parts are wished to be parallel to each other when the green compact having the leg parts and the undercut part is formed, the through-hole is made to have a pair of parallel parts extending from the base part, a part of the through-hole facing the base part is cut off, and thereby, a second sintered compact having a pair of parallel leg parts connected continuously at the base part can be formed. According to the manufacturing method of the present invention, since the deformation such as the warpage or the like does not occur in the thin part around the through-hole in the sintering described above, an excellent parallel state can be given to the leg parts.
Further, the manufacturing method of this invention is characterized in that it comprises a step of retracting and storing the material powder, which is punched out in said step of punching out a shape of said through-hole by inserting a punch-out pin into said preform, outward of the cavity; a step of refilling the stored material powder into said through-hole from which said punch-out pin has been pulled out in said step of pulling out the punch-out pin from the green compact; and a step of removing the material powder refilled into said through-hole after said step of taking out said green compact from said cavity.
In this manufacturing method, if the through-hole is kept as a hollow after the punch-out pin is pulled out from the green compact obtained by further pressurizing and compressing the preform, the thin part may break when the green compact is taken out Therefore, when the punch-out pin is inserted into the preform to punch out the shape of the through-hole, the punched-out material powder is retracted to the outside of the cavity by the punch-out pin and stored. At the time when the punch-out pin is pulled out from the green compact obtained by the pressurization and compression, the material powder retracted as described above is refilled into the through-hole from which the punch-out pin has been pulled out. Since the retracted material powder is temporarily compressed as described above, it can be refilled into the through-hole easily without losing its shape.
When the green compact is taken out thereafter, the green compact can be taken out without causing breakage because its thin part is reinforced by the material powder filled in the through-hole. The taken-out green compact can then become a complete product by removing the material powder refilled in the through-hole.
The material powder punched out as described above may be collected and recycled instead of being refilled into the through-hole.
The manufacturing method according to the present invention in which the punched-out material powder is refilled into the through-hole as described above can be realized more advantageously by a device for manufacturing the green compact, comprising:
a die having in a vertical direction a die hole in a shape matching a contour of the green compact including a through-hole at a part connected continuously to a base part;
the lower punch inserted from an underside of the die into the die hole and forming a cavity to be filled with the material powder together with the die;
an upper punch inserted from an upside of the die into the die hole and pressurizing and compressing the material powder filled in the cavity together with the lower punch;
a punch-out pin having a sectional shape corresponding to a cross-section of the through-hole, provided in the die to face the cavity, and inserted into a preform formed by temporarily compressing the material powder by means of both of the upper and lower punches to punch out a shape of the through-hole; and
a refill pin having a sectional shape corresponding to the cross-section of the through-hole, provided in the die to face the cavity and the punch-out pin, moving backward in synchronization with forward movement of the punch-out pin when the punch-out pin is inserted into the preform, and moving forward in synchronization with backward movement of the punch-out pin when the punch-out pin is pulled out from the preform to refill the material powder into the through-hole.
According to the manufacturing device of the present invention, when the punch-out pin is inserted into the preform to punch out the shape of the through-hole, the refill pin provided to face the punch-out pin moves backward in synchronization with the forward movement of the punch-out pin. As the result, the material powder in the shape of the through-hole which has been punched out by the punch-out pin is sandwiched between the punch-out pin and the refill pin, retracted to a position where the refill pin has been before moving backward, and stored as it is.
Then, when the punch-out pin is pulled out from the preform, the refill pin moves forward in synchronization with the backward movement of the punch-out pin. As the result, the material powder stored in the position where the refill pin has been before moving backward moves while being sandwiched between the punch-out pin and the refill pin so as to be refilled into the through-hole.